四川盆地西部雷口坡组非热液成因鞍形白云石的发现及意义

鞍形白云石通常由热液作用形成，四川盆地西部雷口坡组广泛发育的鞍形白云石也被许多学者归为热液作用成因，并由此将雷口坡组白云岩储层归为热液成因储层。为了进一步认识这些鞍形白云石的成因，并为储层研究提供指引，本文通过对四川盆地西部雷口坡组鞍形白云石的岩石学及地球化学特征分析，较深入地开展了雷口坡组鞍形白云石研究。研究指出区内雷口坡组存在两种成因类型的鞍形白云石；构造裂缝内的鞍形白云石在阴极射线下发亮桔红色光或不发光，氧同位素值相对较高、富集稀土元素、并具负Eu异常、锶-钾-铁-锰含量较高，明显不同于围岩，可能是深部与中酸性岩浆活动有关的岩浆期后热液作用的产物；灰岩中的鞍形白云石斑块、白云岩溶蚀缝孔洞内鞍形白云石沉淀物在阴极射线下发玫瑰红色光，氧同位素值相对较低、贫稀土元素、缺乏Eu异常、锶-钾-铁-锰含量较低，与白云岩围岩极其相似，代表了白云石化流体及物质来源于地层本身，是非热液成因的鞍形白云石，是中-深埋藏条件下，白云岩地层内部流体白云石化作用的产物。区内雷口坡组热液成因鞍形白云石发育分布有限，广泛发育分布的是非热液成因鞍形白云石，指示了雷口坡组储层并非热液作用成因；埋藏条件下，地层流体及热液流体活动较弱，大量的有效储集空间得以保存下来。

Discovery of non-hydrothermal saddle-shaped dolomite in Leikoupo Formation,Western Sichuan Basin and its significance

Since saddle-shaped dolomites are usually formed by hydrothermal fluid, multiple scholars have considered the saddle-shaped dolomite in the Leikoupo Formation, Western Sichuan Basin, as hydrothermal origin and ascribed the reservoir therein as a product of hydrothermal process. The study made an in-depth analysis of the petrology and geochemistry of saddle-shaped dolomite in the area, in order to further identify the origin of the dolomite and the genetic mechanism of the dolomite reservoir. Two origin types of saddle-shaped dolomite were identified-the hydrothermal saddle-shaped dolomite and the non-hydrothermal one. The former developed inside tectonic fractures emits bright orange red light or is non-luminant under cathodoluminescence, featuring relatively high content of δ¹⁸O and Sr-K-Fe-Mn elements, rich rare-earth elements, and negative-Eu abnormality. It's quite different from the matrix rocks, and may be a product of post-magmatic hydrothermal processes following intermediate to acidic magma activities in depth. However, the latter is a product of the dolomitization of medium-to-deep fluids inside the dolomite reservoir. Its saddle-shaped dolomite plaques in limestone and the saddle-shaped dolomite precipitates in dissolved pores or cavities in dolomite emit rose-bengal light under cathodoluminescence, featuring relatively lower content of δ¹⁸O and Sr-K-Fe-Mn elements, as well as deficiency in rare-earth element and Eu abnormality. They are quite similar to the surrounding dolomites, indicating that dolomitization fluids and substance have originated from within the same reservoir. The fact of limited distribution of the saddle-shaped dolomite of hydrothermal origin and wide distribution of the saddle-shaped. dolomite of non-hydrothermal origin in the Leikoupo Formation, indicates that the Leikoupo reservoir is not of hydrothermal origin. The relatively subdued activities of formation and hydrothermal fluids in burial have led to good preservation of effective reservoir space.